1. Field of the Invention
This invention relates to a ball screw which has a nut member engaged with a screw shaft via balls, and which is adapted to convert a rotational movement of a motor into a linear movement in a work table of, for example, a machine tool, and more particularly to an improvement in a ball screw of the type which has an endless ball circulating passage formed by fixing substantially U-shaped return pipes to the nut member.
2. Description of the Related Art
Various types of ball screws having an endless ball circulating passage formed by using so-called return pipes are known. These ball screws, as shown in FIG. 14, are provided with a screw shaft 1 having a helical ball rolling groove 10, a nut member 2 having a helical loaded rolling groove opposed to the ball rolling groove 10 and engaged with the screw shaft 1 via balls 3, and return pipes fixed to the nut member 2 and forming an endless circulating passage for the balls 3.
As shown in FIG. 15, each of such return pipes 4 is formed to a substantially U-shaped cross section, and provided with a pair of leg portions 40 inserted into the nut member 2, and a communication passage portion 41 connecting these leg portions 40 together so that balls 3 can roll in the interior thereof from one leg portion 40 toward the other 40. The nut member 2 is provided with pairs of ball passing holes used to insert the leg portions 40 of the return pipes 4 thereinto, in such a manner that each pair of ball passing holes sandwich the axis of the nut member 2 therebetween. These ball passing holes are formed tangentially with respect to an inner circumferential surface of the nut member. These ball passing holes are formed at intervals of several turns of the loaded rolling groove. The return pipes 4 are formed so that, when the leg portions 20 thereof are inserted into these ball passing holes, the leg portions 20 slightly project into the inner circumferential surface of the nut member 2 and scoop up the balls 3 from the ball rolling groove 10 of the screw shaft 1 into the interior of the return pipes 4. Therefore, when the balls 3 which have rolled as they impart a load to the screw shaft 1 between the ball rolling groove 10 of the screw shaft 1 and loaded rolling groove of the nut member 2 reach the positions in which the leg portions 40 of the return pipes 4 project, the balls 3 are released from the load and leave the ball rolling groove 10 of the screw shaft 1. The balls 3 then roll in an unloaded condition in the interior of the return pipes 4, and are returned to the portions of the ball rolling groove 10 which are several turns backward. Namely, when the return pipes 4 are fixed to the nut member 2, an endless circulating passage for the balls 3 is formed.
In an effective method of smoothly scooping the balls from the ball rolling groove of the screw shaft into the return pipes, the balls are scooped up in the ball rolling direction, i.e., in the direction in which the helical ball rolling groove is stretched radially. In order to practice this method, it is necessary that the leg portions 40 of the return pipes 4 be inclined as shown in FIG. 15 from the cross sections thereof which are perpendicular to the screw shaft toward the axis of the screw shaft by an angle equal to a lead angle xcex1 of the ball rolling groove 10. Since the return pipes 4 are fixed to the nut member 2 so as to stride over the axis of the same, it is necessary that the directions in which each pair of leg portions 40 are inclined at an angle equal to a lead angle xcex1 of the ball rolling groove 10 be opposite to each other, i.e., it is necessary that each pair of leg portions 40 be formed at both ends of the relative communication passage portion 41 so as to be in a twisted state with respect to each other and not in a parallel-extending state.
However, in view of the shape of the return pipes in the related art ball screw, it is physically difficult to insert the leg portions of the return pipes in order into the ball passing holes of the nut member. The inserting of each pair of leg portions into the ball passing holes had to be done at the same time by bringing each return pipe close to the nut member in the direction (from an upper portion of FIG. 14) perpendicular to the axis of the nut member. In order to practice this method, it is necessary that the ball passing holes into which the leg portions of the return pipes are inserted of the nut member be formed perpendicularly with respect to the axis of the nut member. Since the leg portions of the return pipes are inclined at an angle equal to the lead angle of the ball rolling groove, an inner diameter of the ball passing holes had to be set larger than an actual outer diameter of the return pipes.
Therefore, in the related art ball screw, the return pipes cannot be positioned accurately by merely inserting the leg portions thereof into the ball passing holes of the nut member. It was necessary that the return pipes were positioned manually as the balls were rolled in practice after the leg portions had been inserted into the ball passing holes. Moreover, since the inner diameter of the ball passing holes is larger than the outer diameter of the return pipes, there is a fear that dust enter the clearances between these holes and pipes to cause the smooth circulation of the balls to be early spoiled.
The inner circumferential surface of the nut member is provided with a loaded rolling groove formed at a predetermined lead angle. Therefore, when ball passing holes of a large inner diameter are made in the direction perpendicular to the axis of the nut member, there is a fear that such ball passing holes interfere with adjacent portions of the loaded rolling groove. Especially, when a lead length of this rolling groove was small with the diameter of the balls large, it was difficult to form the ball passing holes.
The present invention has been made in view of the above circumstances, and provides a return pipe type ball screw capable of accurately positioning and fixing return pipes on and to a nut member even when a troublesome positioning operation is not carried out, and circulating balls smoothly and endlessly for a long period of time without the possibility that dust enters the interior of the nut member.
The present invention also provides a ball screw capable of circulating balls endlessly through return pipes even when a lead length of a helical ball rolling groove is small with a diameter of the endlessly circulating balls large.
The ball screw according to the present invention includes a screw shaft provided with a helical ball rolling groove having a predetermined lead angle; a nut member having a through hole through which the screw shaft is passed, and a helical loaded rolling groove formed in an inner circumferential surface of the through hole, opposed to the ball rolling groove and engaged with the screw shaft via plural balls rolling in the ball rolling groove and loaded rolling groove; a pair of ball passing holes formed so as to extend through the nut member, each of which is joined to opposed side portions of the loaded rolling groove; and a return pipe having a pair of leg portions inserted into relative pair of ball passing holes, and fixed to the nut member so as to form an endless ball circulating passage, wherein the ball passing holes are formed so as to incline from the cross sections thereof which are perpendicular to the axis of the screw shaft in the axial direction of the screw shaft by an angle equal to the lead angle of the ball rolling groove of the screw shaft, the return pipe has a pair of return pieces adapted to be fitted in relative ball passing holes.
In the ball screw according to the present invention, the ball passing holes formed in the nut member so as to make the balls, which have rolled as they impart a load to the screw shaft between the loaded rolling groove of the nut member and ball rolling groove of the screw shaft, enter the return pipes smoothly are inclined from cross sections thereof which are perpendicular to the axis of the screw shaft in the axial direction thereof at an angle equal to the lead angle of the ball rolling groove of the screw shaft. Since each of the return pipes is formed by combining therewith a pair of return pieces adapted to be fitted in the relative ball passing holes, the return pieces can be inserted and fixed separately in and to the ball passing holes of the nut member. Therefore, the ball passing holes of the nut member into which the return pieces are inserted may have an inner diameter equal to the outer diameter of the return pieces, and such return pieces can be positioned accurately by merely inserting the same into the ball passing holes of the nut member. The ball passing holes of the nut member may be formed to a size which enables the return pieces to be fitted therein without causing a clearance to occur between the holes and return pieces. Since a clearance does not occur between such ball passing holes and return pieces, the entry of dust into the interior of the nut member can be prevented.
The ball passing holes made in the nut member may have a size large enough to enable the return pieces to be fitted therein without causing a clearance to occur between these holes and return pieces. Therefore, the ball passing holes do not interfere with adjacent portions of the loaded rolling groove, and, even when a lead length of the ball rolling groove is small with the diameter of the balls large, an endless ball circulating passage can be formed by the return pipes.
As described above, in the ball screw according to the present invention, the pairs of ball passing holes in the nut member are formed so as to incline in the axial direction of the nut member by an angle equal to the lead angle of the ball rolling groove, and each of the return pipes inserted into the relative ball passing holes is formed by combining a pair of return pieces therewith. Therefore, the ball passing holes in the nut member into which these return pieces are inserted serve the purpose when these holes have an inner diameter equal to the outer diameter of the return pieces. The return pieces can be accurately positioned by merely inserting the same into the ball passing holes separately. This enables the return pipes to be positioned and fixed accurately with respect to the nut member without carrying out a troublesome positioning operation. Since a clearance does not occur between the ball passing holes and return pieces, there is not a fear that dust enters the interior of the nut member, and the balls can be circulated endlessly and smoothly for a long period of time.
Since the ball passing holes made in the nut member may be set to a size large enough to enable the return pieces to be fitted therein without causing a clearance to occur between the same holes and return pieces, the ball passing holes do not interfere with adjacent portions of the loaded rolling groove. Even when the lead length of the ball rolling groove is small with the diameter of the endlessly circulating balls large, a ball screw can be formed by using a return pipe system.